Corrosion resistant coatings



United States Patent 3,198,673 CORRQSION RESKSTANT COATINGS Kenneth J. Dooley, Middletown, and Hugh L. Thompson,

Merideu, Conn, assignors to The Dir-Lite Chemical Corporation, Middletown, (Iona, a corporation of Connecticut No Drawing. Filed lune 27, 1961, Ser. No. 119,795

5 Claims. (ill. 148-6.15)

This invention relates to the production of phosphated black oxide coatings, and relates more particularly to the production on ferrous metal parts of such coatings in which a major proportion of the coating is black iron oxide and a minor proportion is iron phosphate, these two components being in intimate molecular admixture.

In producing the coatings of this invention, the ferrous metal part is first coated with black oxide by any suitable one of numerous conventional methods. The thus coated metal part is then briefly subjected at a moderate temperaproduce no material dimensional change of the treated piece. Black oxide coatings are, therefore, particularly suited for moving parts. However, their wear resistance is not of the best, and their corrosion resistance leaves much to be desired. Another disadvantage of the conventional coatings is that, particularly when applied to cast or malleable iron or stainless steel, they are apt to be characterized by the presence of a black smut which does not adhere properly to the metal surface.

The phosphatizing treatment of this invention eliminates any non-adherent black smut that may be present in the initial black oxide coating, and gives the coating a deeper and more desirable black color. As compared to conventional black oxide coatings, the phosphated black oxide coatings of this invention have enhanced wear resistance and greatly increased corrosion resistance. The disadvantages of conventional black oxide coatings are thus overcome by this invention. Furthermore, our phosphated coatings possess the unique property of being capable of firmly bonding rubber coatings vulcanized thereto in any usual manner, whereas such rubber coatings will not adhere to conventional black oxide coatings.

It is well known that the corrosion resistance of a conventional black oxide coating may be greatly increased by oiling the coated piece with any of a number of lubricating or other oils. However, for many commercial uses, an oil film on the treated piece is very objectionable. For example, many metal parts used in cameras, clocks, optical equipment, typewriters and other business machines, and radio and television sets, cannot tolerate an oily coating. On some such parts, a dry wax film may be employed, but this provides relatively little protection because the wax film is easily broken, and once it has been broken, rusting can take place. The phosphated black oxide coatings of this invention may be oiled if desired, but they are especially adapted for use where a dry finish is required, and require no protective wax film.

The particular manner of producing the initial black oxide coating is immaterial to this invention. Three classes of suitable methods are set forth in United States Military Specification MIL-C-13924A, dated 26 December 1956. The alkaline oxidizing process is especially suited for cast iron, malleable iron, plain carbon steels and low alloy steels. Either the alkaline chromate oxidizing process or the fused salt oxidizing process may be used for corrosion resistant steels, usually called stainless steel, and the latter process may also be used for carbon steels having draw temperatures above 900 F. The ferrous metal parts to whichthis invention is applicable may fall in any of the foregoing categories and may be coated with black oxide by any appropriate method, including various known methods other than the three just mentioned.

Parts blackened in accordance with the foregoing Military Specification or with any of the conventional methods of producing black oxide coatings, will seldom withstand more than one-half hour of a standard salt spray test. The phosphated black oxide coatings of this invention, on the other hand, will withstand one and one-half hours of a standard salt spray test without showing any rust spots. Furthermore, the coatings of this invention are considerably more resistant to bufling and other forms of mechanical abrasion than conventional black oxide coatings.

Many different types of phosphatizing solutions are usable in this invention, some of which have their special advantages and disadvantages. The choices of the acidic phosphatizing agent and the volatile solvent are, of course, mutually dependent, since the former must be soluble in the latter to an extent sufficient to produce the desired dilute treating solution. The phosphatizing agent. may be phosphoric acid or an acid salt or ester of phosphoric acid. The solvent may be water or a volatile organic solvent.

Where the phosphatizing agent is phosphoric acid, the solvent may be water, and the solution may contain from 0.1% to 10% of the commercial acid, known as the syrup, by volume based on the water. As is well known to those skilled in the art, a conventional black oxide coating tends to reproduce the surface finish of the ferrous part to which it is applied. A dense, highly polished black om'de surface will respond to a weaker acid solution than a comparatively rough and more porous black oxide coating. In either case, the acid must not be too concentrated, in order to avoid excessive attack and dissolving of the black oxide coating. When water in the solvent, how: ever, the best appearing finish is not obtained, careful prior cleaning of the oxide. coated part is required in order to obtain complete and uniform wetting of its surface, and the necessary subsequent drying is relatively slow and costly. In order to avoid these disadvantages of water solutions, it is preferred to use a volatile organic solvent for the phosphatizing solution.

With commercial phosphoric acid syrup, any of the volatile organic solvents described in Verner et al. Patent 2,515,934, issued July 18, 1950, may be used to make up a phosphatizing solution for use according to this'invention, The preferred phosphatizing liquid of that patent is a mixture of equal volumes of acetone and carbon tetrachloride, in which is dissolved 3% of commercial phosphoric acid syrup, and this solution is eminently suited for use in this invention. As stated in that patent, this solution, because of the presence of the carbon tetrachloride, has a flash point slightly above that of ordinary kerosene, and may therefore safely be handled, with only the usual precautions employed in paint spraying op er-ations.

Because of the necessary drying step of the process of this invention, the organic solvent employed must be volatile and, for practical reason, itshould be relatively inexpensive and reasonably free from fire and health hazards. For the purposes of this invention, We

I regard as non-volatile, any solvent which, when coated C? on a part by dipping, will not evaporate to a completely dry surface upon air drying at room temperature for not more than thirty hours. A much shorter time is, of course, highly desirable. Under this definition, all lubricating oils, even a light spindle oil, are non-volatile. Where a dry finish is required on the part, the phosphatizing solution must be non-oily, by which we mean that it must not leave any oily residue. On the other hand, where an oily finish is desired, there would be no objection to such a residue, and in such case, the phosphatizing solution could contain a small proportion of a suitable lubricant dissolved in the organic solvent, as an alternative to oiling the part after drying.

Commercial phosphoric acid syrup is, unfortunately, not sutficiently soluble for the purposes of this invention, in many of the organic solvents commonly used in industry. There are, however, many commercially obtainable acid salts or esters of phosphoric acid that are soluble in those solvents. We prefer to employ a soluble ester that is quite strongly acid, such as the readily available monoalkyl acid phosphates (general formula, RH PO dialkyl acid phosphates (general formula R HPO and dialkyl acid pyrophosphates (general formula, RzHzPgOq). We shall refer to these compounds generically as alkyl acid phosphates.

Among the common organic solvents of appropriate volatility which dissolve alkyl acid phosphates but not phosphoric acid syrup, are toluol, xylol, the petroleum distillates ranging, in order of increasing flash point, from No. 1, petroleum ether, to No. 5, petroleum naptha, and certain chlorinated solvents. T oluol, xylol and the lower numbered petroleum distillates present an excessive fire hazard, unless used with a flash inhibitor, such as the carbon tetrachloride of the foregoing example, which ads considerably to their cost and sometimes presents a health hazard. Because of its cheapness and the safety in use resulting from its comparatively high flash point, we prefer to use petroleum naphtha, which is generally known in the trade as Stoddard Solvent. If complete non-inflammability. is required, chlorinated solvents may be employed, such as trichloroethylene, methylene chloride, trichloroethane and perchloroethane. However, the chlorinated solvents are not only relatively expensive but present possible health hazards and, from these standpoints, are less desirable the petroleum naphtha.

With the preferred petroleum naphtha as the volatile solvent, we may use any of the following alkyl acid phosphates: the tridecyl and the lauryl monoalkyl acid orthophosphates; the iso-octyl and the 2-ethyl hexyl dialkyl acid orthophosphates; and the iso-octyl and the 2- ethyl hexyl dialkyl acid pyrophosphates. We have found two and one-half percent (2 /z%) of the alkyl acid phosphate to ninety-seven and one-half percent (97 /2%) by volume of the petroleum naphtha, to be a satisfactory proportion for general use, although this concentration is by no means critical. We have found no detectable variations in the results obtained with any of the foregoing phosphatizing solutions at the stated concentration, despite the fact that the solutions have various degrees of acidity as a result of the differing acid numbers of the alkyl acid phosphates.

' There are a number of other alkyl acid phosphates which are not sufi'iciently soluble in petroleum naphtha, but are sufiiciently soluble in others of the volatile organic solvents listed above, such as the chlorinated solvents. Among such monoalkyl acid orthophosphates are iso-amyl, iso-octyl, Z-ethyl hexyl and decyl; among the dialkyl acid orthophosphates are butoxyethyl, n-butyl and which the part is dried to remove the volatile solvent. The phosphatizing solution may be applied to the coated part either by dipping or spraying, preferably at room temperature. The solution must be weakly acidic, the temperature must be low, and the time of treatment must be short, say five seconds to ten minutes, or else the black oxide coating may be attacked and dissolved. The purpose of these mild conditions of treatment is to convert the black oxide coating to a limited extent into a phosphate coating, while avoiding destruction of the black oxide coating. Only a minor proportion of the black oxide is converted into iron phosphate, and the conversion probably takes place principally at the surface of the initial black oxide coating. The time of treatment is, of course, affected by the acid strength of the phosphatizing solution, and its temperature. The stronger the acidity of the solution, the shorter the time of treatment must be in order to avoid an excessive attack on the oxide. With the alkyl phosphatizing solutions described above, at room temperature, a contact time of five minutes will show no attack, Whereas, in some cases, an immersion of ten minutes may cause an attack on the oxide located at the edges and corners of a lightly oxidized piece of steel.

With the conventional methods of producing the initial black oxide coating, the final step is a water wash to remove all traces of processing salts. At the conclusion of this washing step, the parts are completely covered with a film of water. It is customary next to dry the parts, but this drying step may be avoided by immersing the moist parts directly in the phosphatizing solution. Where the solvent of the phosphatizing solution is water, this presents no difiiculty whatever. Where a volatile organic solvent is employed, such direct immersion becomes possiblc upon adding to the phosphatizing solution a substance that causes it to become highly hydrophobic or water displacing. For producing this water displacing property, we have found the glycols and glycol derivatives to be effective. For example, hexylene glycol or ethylene glycol monobutyl ether may be added to the phosphatizing solution in the proportion of from 0.25% to 2.0%, based on the volume of the solution before the addition. These water displacing compounds cause the phosphatizing solution to displace any water present on the parts which are immersed therein, so that all surfaces of the parts are immediately exposed to the action of the phosphatizing agent. The water thus displaced will fall to the bottom of the treating tank and may be drawn off from time to time.

in order to afford a direct comparison of rust resistance between the phosphated coatings of this invention and sevcralprior art coatings, various specimens were prepared as described below and then subjected to a standard humidity cabinet test. Nine like pieces were cut from the same cold rolled steel. Specimens Nos. 1 and 2 were withdrawn at this stage. The remaining pieces were cleaned by vapor degreasing so as to leave a surface suitable for plating, and then dried. These pieces were the nimmersed for 15 minutes in a conventional aikaline oxidizing bath, the processing salts of this bath being those commercially sold under the name DuLite Oxiblak by the Du-Lite Chemical Corporation, Middletown, Connecticut. The pieces were then washed in running water until free of alkali, and then dried. Specimens 3 to 7 were Withdrawn at this stage. The remaining two pieces, Specimens 8 and 9, were then immersed in a phosphatizing solution consisting of one part by volume of mono 2-ethyl hexyl acid orthophosphate, and ninety-nine parts by volume of petroleum naphtha. The petroleum naphtha was one having a flash point above 100 F., a distillation end-point about 350 F., and a kauri-butanol number higher than 30. The parts were removed from the solution after five minutes, immersion, and the solvent evaporated in a forced current of air at room temperature.

These nine specimens were delivered to an independent testing laboratory for testing in a standard humidity cabinet at 120 F. and 95-100% relative humidity. The laboratory was also supplied with two oils, which we shall designate A oil and B oil. The A oil was a light, neutral lubricating oil, commercially known as spindle oil. The 13 oil consisted of 98 parts by volume of the same spindle oil and two parts by volume of the same alkyl acid phosphate used in phosphatizing Specimens 8 and 9. The laboratory was instructed to, and

reported that it did, treat the samples as follows: Specimens 1, 4 and 5 were cleaned in petroleum ether, dipped in the A oil, drained and placed in the humidity cabinet. Specimens 2, 6 and 7 were cleaned in petroleum ether, dipped in the B oil, drained and placed in the cabinet. Specimens 3, 8 and 9 were placed in the cabinet dry, as received by the laboratory.

The results of the humidity cabinet tests were reported as follows: At theend of 24 hours, there was slight rust on Specimens 1 and 2 and moderate rust on Specimen 3, whereas the other specimens showed no rusting. At the end of 144 hours, there was no rust on any of these other specimens. At the end of 168 hours, there was very slight rust on Specimen 8 and moderate rust on Specimen 9. At the end of 600 hours, the test was discontinued, this being considered indefinite life, and Specimens 4, 5, 6 and 7 still showed no rusting.

It will be observed from the foregoing that the dry phosphated black oxide coatings of this invention (Specimens 8 and 9) showed more than six times the rust resistance of the oiled steel and the dry black oxide coating (Specimens 1, 2 and 3). The oiled black oxide coatings all showed indefiinite rust resistance, and there was no observable difference in this regard whether the coatings were oiled with plain spindle oil (Specimens 4 and 5), or spindle oil containing the alkyl acid phosphate (Specimens 6 and 7).

Although we have thus described our invention in considerable detail in the best form of which We are aware, in accordance with the patent statutes, it will be evident that various changes and modifications may be made by those skilled in the art without departing from the spirit of our invention. Accordingly, We desire to be limited only by the scope of the appended claims.

We claim:

1. A method of producing on a ferrous part a deep black, smut-free, corrosion resisting coating that produces no material dimensional change of the ferrous part, said coating consisting essentially of a major proportion of black iron oxide and a minor proportion of iron phosphate in intimate molecular admixture, which comprises coating the ferrous part with black iron oxide, treating the coated part with a solution consisting essentially of a volatile solvent and an acidic phosphatizing agent, and drying the part to remove the volatile solvent, the conditions of said treatment being sufliciently mild so that said black iron oxide coating is not destroyed and is only partially converted into iron phosphate.

2. The method claimed in claim 1 in which the volatile solvent is water and the acidic phosphatizing agent is phosphoric acid syrup.

3. The method claimed in claim 1 in which the volatile solvent is an organic solvent and the acidic phosphatizing agent is an alkyl acid phosphate.

4. The method claimed in claim 1 in which the volatile solvent is a light petroleum distillate and the phosphatizing agent is mono 2-ethyl hexyl orthophosphate.

5. A method of producing on a ferrous part a deep black, smut-free, corrosion resisting coating that produces no material dimensional change of the ferrous part, said coating consisting essentially of a major proportion of black iron oxide and a minor proportion of iron phosphate in intimate molecular admixture, which cornprises processing the ferrous part to produce thereon a coating of black iron oxide, the final step of said processing being that of washing the coated part with water to remove the excess coating materials, dipping the coated part while still moist with wash water in a solution consisting essentially of a volatile organic solvent, an alkyl acid phosphate and a hydrophobic compound, whereby the water present on the part will be removed therefrom and form a separate liquid phase, and drying the part to remove the volatile solvent, the conditions of said treat ment being sufiiciently mild so that said black iron oxide coating is not destroyed and is only partially converted into iron phosphate.

References Cited by the Examiner UNITED STATES PATENTS 1,167,966 1/16 Allen 148-6.15 2,217,586 10/40 Zapf 148-615 2,437,441 3/48 Rogers 1486.15 2,515,934 7/50 Verner 1486.l5 2,774,701 12/56 Koryta 1486.15 2,809,907 10/57 Cramer l486.15 2,813,813 11/57 Ley et a1. 1486.15 2,920,019 1/60 Smith et a1. 1486.15

FOREIGN PATENTS 396,746 8/33 Great Britain.

607,061 10/60 Canada.

References Jilted by the Applicant UNITED STATES PATENTS 1,761,186 6/30 Baker et al. 2,080,299 5/ 37 Benning et a1. 2,789,070 4/57 Copelin. 2,817,610 12/57 Newell et al.

FOREIGN PATENTS 735,933 8/55 Great Britain.

RICHARD D. NEVIUS, Primary Examiner. MARCUS U. LYONS, Examiner. 

1. A METHOD OF PRODUCING ON A FERROUS PART A DEEP BLACK, SMUT-FREE, CORROSION RESISTING COATING THAT PRODUCES NO MATERIAL DIMENSIONAL CHANGE OF THE FERROUS PART, SAID COATING CONSISTING ESSENTIALLY OF A MAJOR PROPORTION OF BLACK IRON OXIDE AND A MINOR PROPORTION OF IRON PHOSPHATE IN INTIMATE MOLECULAR ADMIXTURE, WHICH COMPRISES COATING THE FERROUS PART WITH BLACK IRON OXIDE, TREATING THE COATED PART WITH A SOLUTION CONSISTING ESSENTIALLY OF A VOLATILE SOLVENT AND AN ACIDIC PHOSPHATIZING AGENT, AND DRYING THE PART TO REMOVE THE VOLATILE SOLVENT, THE CONDITIONS OF SAID TREATMENT BEING SUFFICIENTLY MILD SO THAT SAID BLACK IRON OXIDE COATING IS NOT DESTROYED AND IS ONLY PARTIALLY CONVERTED INTO IRON PHOSPHATE. 